SpamSieve 3.0.1’s Check inboxes for new messages not sent to Mail extension feature can slow down Apple Mail if there are lots of messages in the inbox (or in another mailbox that you’ve selected for filtering), as Mail gets bogged down when SpamSieve asks it to read the list of messages. SpamSieve has limits in place where it will skip filtering if it looks like there are enough messages to cause a problem, but perhaps you’ve raised the limit or there are so many messages that simply counting them to see whether the limit applies is stressing Mail.
SpamSieve 3.0.2 (now in beta) checks for new messages in a different way that’s much faster and lower overhead (if you give SpamSieve Full Disk Access). So, first I would suggest updating to the beta.
If that doesn’t help, you could record sample reports from Mail and SpamSieve to see what they are doing during the hang.
SpamSieve doesn’t interact with Spark, so it wouldn’t be directly causing it to hang. Perhaps it’s a side effect of Mail getting bogged down. Or you could sample it, too, in case there’s some other common factor that’s affecting both apps.