I'm trying to load old wallet (which I have not been opening for months) and it takes too much time (~minutes). In this time seems like I can't do any further requests to RPC. After this new calls to this wallet takes milliseconds. Is it true, that Bitcoin somehow synchronize old wallet with the current state of the network and that's why it takes so much time?
Is it true, that Bitcoin somehow synchronize old wallet with the current state of the network
Yes. If the wallet file is not synced with the current blockchain state, a rescan will occur starting from the block that is recorded in the wallet file. It does not rescan all of history, only the things that it has not yet seen.
and that's why it takes so much time?
Perhaps. There are other things that can effect wallet loading times. In particular, if you have a lot of keys and transactions, loading the wallet could take a long time too.