It happpens under various circumstances, nothing easy to pin-point here.
Example: on two identical iMacs (4 cores, 16GB RAM, PCIe SSD) with about 100GB of local synced IMAP and the same accounts, it's slow for one user's iMac but fast on the other. Deleted TB and profile, fresh user, fresh TB install, fresh config; same result.
Example: on two identical iMacs (4 cores, 16GB RAM, PCIe SSD) with about 100GB of local synced IMAP and the same accounts, it's slow for one user's iMac but fast on the other. Deleted TB and profile, fresh user, fresh TB install, fresh config; same result.