I have read that Bisq is slow. Is this true? Any way to rectify it?
It's slow in the sense that you're doing P2P trades, so after paying, you have to wait for the seller to confirm the payment and 'release the funds' to make them available to you. In my experience that usually takes a few hours, especially if you take an offer during work hours. If the seller is not cooperating, you may also have to wait till the end of the trading period (7 days) to resolve the issue with a mediator.
However, the benefits of using this software
heavily outweigh this 'speed' limitation.
The slow 'speed' cannot be avoided without making the whole thing custodial / trust-based. Keep in mind Bisq isn't even a company; but that's the cool thing, there's no central server, no central CEO to bribe or anything like this. Your PC, while staying on,
is the server keeping your offer online.
It's also worth mentioning many of the fiat payment methods that are offered, are pretty fast, like Revolut, Strike and Instant SEPA.