On startup the GUI needed considerable time to "load" my default watch-only wallet (21954 descriptors or so-called Patoshi blocks and some tenthousands of transactions) and show the wallet's overview/balance and be able to display the wallet's transactions. I left Bitcoin-Qt do its thing while browsing with Firefox and didn't measure after what time the GUI was usable.
In fact, from debug.log it takes only ~5.5s to load the wallet, but considerably longer if you ask for the final balance and display a list of all known transactions.
If of interest, here are some parts from debug.log from the Bitcoin-Qt startup:
2024-09-26T14:25:26Z InitParameterInteraction: parameter interaction: -bind set -> setting -listen=1
2024-09-26T14:25:26Z InitParameterInteraction: parameter interaction: -proxy set -> setting -upnp=0
2024-09-26T14:25:26Z InitParameterInteraction: parameter interaction: -proxy set -> setting -natpmp=0
2024-09-26T14:25:26Z InitParameterInteraction: parameter interaction: -proxy set -> setting -discover=0
2024-09-26T14:25:26Z InitParameterInteraction: parameter interaction: -onlynet excludes IPv4 and IPv6 -> setting -dnsseed=0
2024-09-26T14:25:26Z Qt 5.15.11 (static), plugin=xcb (static)
2024-09-26T14:25:26Z Static plugins:
2024-09-26T14:25:26Z QXcbIntegrationPlugin, version 331520
2024-09-26T14:25:26Z Style: fusion / QFusionStyle
2024-09-26T14:25:26Z System: Ubuntu 22.04.5 LTS, x86_64-little_endian-lp64
...
2024-09-26T14:25:26Z scheduler thread start
...
2024-09-26T14:25:30Z Cache configuration:
2024-09-26T14:25:30Z * Using 2.0 MiB for block index database
2024-09-26T14:25:30Z * Using 56.0 MiB for transaction index database
2024-09-26T14:25:30Z * Using 49.0 MiB for basic block filter index database
2024-09-26T14:25:30Z * Using 8.0 MiB for chain state database
2024-09-26T14:25:30Z * Using 335.0 MiB for in-memory UTXO set (plus up to 976.6 MiB of unused mempool space)
2024-09-26T14:25:30Z init message: Loading block index…
2024-09-26T14:25:30Z Assuming ancestors of block 000000000000000000026811d149d4d261995ec5b3f64f439a0a10e1a464af9a have valid signatures.
2024-09-26T14:25:30Z Setting nMinimumChainWork=000000000000000000000000000000000000000063c4ebd298db40af57541800
...
2024-09-26T14:25:40Z Verification: No coin database inconsistencies in last 6 blocks (23319 transactions)
2024-09-26T14:25:40Z block index 9954ms
...
2024-09-26T14:25:40Z init message: Loading wallet…
2024-09-26T14:25:40Z [watch_patoshi] Wallet file version = 10500, last client version = 270100
2024-09-26T14:25:42Z [watch_patoshi] Descriptors: 21954, Descriptor Keys: 0 plaintext, 0 encrypted, 0 total.
2024-09-26T14:25:46Z [watch_patoshi] Wallet completed loading in 5521ms
2024-09-26T14:25:46Z [watch_patoshi] setKeyPool.size() = 0
2024-09-26T14:25:46Z [watch_patoshi] mapWallet.size() = 60415
2024-09-26T14:25:46Z [watch_patoshi] m_address_book.size() = 21954
2024-09-26T14:25:46Z Setting NODE_NETWORK on non-prune mode
2024-09-26T14:25:46Z block tree size = 863006
2024-09-26T14:25:46Z nBestHeight = 862958
2024-09-26T14:25:46Z initload thread start
2024-09-26T14:25:46Z Bound to 127.0.0.1:8333
2024-09-26T14:25:46Z Bound to 127.0.0.1:8334
2024-09-26T14:25:46Z Loading 104767 mempool transactions from disk...
2024-09-26T14:25:46Z txindex thread start
2024-09-26T14:25:46Z txindex is enabled at height 862958
2024-09-26T14:25:46Z txindex thread exit
2024-09-26T14:25:46Z Loaded 2 addresses from "anchors.dat"
2024-09-26T14:25:46Z 2 block-relay-only anchors will be tried for connections.
2024-09-26T14:25:46Z init message: Starting network threads…
2024-09-26T14:25:46Z DNS seeding disabled
2024-09-26T14:25:46Z net thread start
2024-09-26T14:25:46Z opencon thread start
2024-09-26T14:25:46Z addcon thread start
2024-09-26T14:25:46Z msghand thread start
2024-09-26T14:25:46Z basic block filter index thread start
2024-09-26T14:25:46Z basic block filter index is enabled at height 862958
2024-09-26T14:25:46Z basic block filter index thread exit
2024-09-26T14:25:46Z coinstatsindex thread start
2024-09-26T14:25:46Z coinstatsindex is enabled at height 862958
2024-09-26T14:25:46Z coinstatsindex thread exit
2024-09-26T14:25:46Z torcontrol thread start
2024-09-26T14:25:46Z Leaving InitialBlockDownload (latching to false)
2024-09-26T14:25:46Z init message: Done loading
...
2024-09-26T14:43:09Z Imported mempool transactions from disk: 104257 succeeded, 84 failed, 0 expired, 426 already there, 0 waiting for initial broadcast
2024-09-26T14:43:09Z initload thread exit
...
For the GUI to be fully responsive, AFAIR it took longer than initload thread exit at 14:43 UTC. While the GUI was still crunching on "loading/preparing" the wallet's details, I could see in debug.log that block sync and wallet's transactions sync was already in normal progress. This node wasn't far from blockchain tip, anyway.