Here is my review for the site.

Right of the bat I kind of disappointed since the mixer did not accept a Bench32 address. Nevertheless, the mixer provides an enjoyable and easy way of mixing. It definitely gives a straightway to mix Bitcoin for the user that is less-tech-savvy since Mixtum doesn't let the user customize any 'technichal things', that would be good for them, In other hands, despite a fixed algorithm that set up within the system, I'd believe some of the users want to customize the mixing process especially the time span of process. I understand that Mixtum has a good reason for them as stated of the https://mixtum.io/#solution, but someone might disagree with that.

Speed, and accuracy. I got the exact value of the mixed coins as calculated on the value calculator that the site provides. It comes with two different tx's, and both of them are received within the time span as stated in the sites(1~6 hours).

User Interfaces, Mixtum does could potentially become the easiest site to mix a coin, a straight-ahead instruction from the start of the website to the point where a user received their mixed coin are as simple as it can.

Website design. I'd personally like the one-page types of the website about the information, and some sort of things. But I'd believe it still could be maximized to be somewhat eye-catchy. Overall, the design does provide accessible information, but I still think it could be maximized so the user could be more interested.
Honestly, the background color over some of the text isn't suitable, IMO. especially on https://mixtum.io/#solution. Maybe try to like don't separate the box within the section so it just likes the other section.

One more thing that bothers upon inspection is.
Aren't that ironic somehow a mixing site that should provide a tool to make Bitcoin become full anonymity are using a questionable service like Cloudflare, and also Google analytics. Don't get me wrong, but those services aren't privacy-oriented.