I recently deposited a 6-figure amount to a Fideltiy trading account. Was reflected in my account "next day". Not sure what went on overnight to verify funds... "Overnight clearing" of funds between banks is most common these days. Requiring "10 days to clear" benefits someone, but not you.
Is IB a FDIC bank or are they banking with someone? If they are a bank they are subject to FDIC rule CC - otherwise their bank is. A 10 day waiting period is unreasonable and probably indicates some poor experience.
Try to get money back out you deposited by ACH. It's a forty-four business day (that's right, 44) hold! Of course you can just trade opposite soon to expire options positions in your IB and another account until the IB position ends up at zero and you've effectively circumvented their "security" feature, so it's a worthless policy to stop true scammers. But it can make it pretty damn difficult for normal people who might not expect such an insanely long hold time and have finally decided to give IB the boot only to have them get one last f-you in on you.
I don't think I've sent a cheque to a broker for 15 years... https://www.bloomberg.com/news/articles/2017-07-26/why-can-t-americans-give-up-paper-checks GAT
Military commissaries are the worst for that! It's funny because I think most of them use checks because they think debit cards aren't "secure", but don't realize that their bank account number and routing number are printed in plain sight on the front of every check!