I would definitely voice my concerns and ask them to speak slower or explain something again If I didn't understand.

Is this situation with a tutor on iTalki or a regular person you meet somewhere? In both cases I think the best would be to ask if they can slow down and speak simpler. In the latter case the point likely isn't in improving your language skills, so effective communication is paramount. It isn't effective if you aren't essentially understanding them (50-60% is so low that you might feel that you're understanding them, but you're missing huge parts and making a lot of incorrect assumptions). In the latter, I would actually quite likely just pretend that I understand. I usually record my lessons (with permission) and I'll edit the recording later so that I can listen to the tutor's part it over and over again. For me this has tremendously helped my listening comprehension and I've been able to raise it from about 70% with any given tutor to around 90-95%. I find that pretending to understand and waiting for cues that can get you back on track is also a quite a normal part of learning a language at a certain level. You can't communicate if the other person has to struggle too much to speak at your level. The trick is to notice where you must get the right information and what can be let slide. I someone says to you "let's go grab a couple of cups of coffee tomorrow at 2 pm, you'll be fine even if you don't exactly know what "grab", "couple", or "cups" means if you just can understand that they are suggesting some activity involving coffee tomorrow at 2 pm, but you'll want to get the time right. I think being able to cope with this type of ambiguity is what sets a successful language learner apart form someone who is too perfectionist and finds it hard to function, if they don't understand each and every word.

I usually just say it, "nope, sorry you lost me there 😅"... And if it happens to you with a teacher, yes, definitely, tell him/her! I always tell my students to interrupt me anytime if they don't understand! :)

I would catch the key words to understand the main idea. I would also ask him/her to speak slowly. I would ask him to explain what I didn't catch by saying " what you mean was... did you you mean... etc" I would also apologize and aske them to speak in a simpler language because I'm still learning

If my intention is to practice speaking then I'll just try to change the topics into those I find more comfortable with. But if I really want to make a connection I'll definitely wish to listen, so I'll just tell them that I do care so "please could you say that again/ rephrase it?"