Second this, but to elaborate, likely it's because they're using chrome when it runs fast and other browsers when it's not. From my perspective, everything Google does is just an effort to get you stuck in their ecosystem.
Everything about Gmail is very confusing to me; I have no love for Outlook (it's absolutely garbage) but in garbage rankings, Outlook is a bit more functional than Gmail is. Outlook has a more complete business eco-system vision I think than Gmail does, and there is a lot better integration with the other elements of Microsoft's ecosystem than Chrome has. For example, when I attach a Sharepoint/OneDrive doc to an email in Outlook, it offers to let me set the permissions for the recipients automatically. This doesn't make up for Sharepoint/OneDrive's awful permissions handling in general, but this element is convenient at least. (Again, I cannot stress how frustrating it is to even __see__ what permissions someone has for a Sharepoint/OneDrive document as regardless of your screen resolution/browser size, you're confined to a few centimeter width sized window, never mind that sometimes saving just outright refuses to work, and bulk-adding persons is even worse)
Gmail has awful design in many places; I don't like that a simple option like FWD is hidden behind the ... menu. I don't like that selecting an email from the list in gmail results in the menu buttons suddenly expanding. Even worse, the location of the reply button when the menu buttons are contracted is the same as the location of the "Mark as Spam" button when expanded; on browsers that aren't chrome, the display lag is enough that I can move my cursor far faster than the menu buttons expand and what __was__ the reply button location is now the "mark as spam" button location.
I don't like that it takes a good 5+ seconds to load the basic email list on a 100 Mbit connection. I don't like that logging into Gmail logs me into every other Google platform (e.g., YouTube) and I start getting spam about different channels or reminders to "engage" with various channels/social media. I don't like that I have to install a .dpkg for Mac to use Gchat video when other apps (e.g., Telegram, Skype, WhatsApp) are just a regular application that I can remove with drag/drop. I really don't like how "foreign" the Gmail/Google theming is with basically everything on MacOS and even on Android, or how few controls over basic UI functionality I get. (I really struggled to find in the Playstore how to monitor the progress of a download...)
Gmail and Google at large feels like an ecosystem that Google just expects everyone to buy into regardless of the platform/experience, and I truly cannot understand their intended way of handling the UI most of the time. The underlying methods of categorizing/indexing the information of course is fine, but actually interacting with it is frustrating for me compared to every Google product contemporary. As much as I dislike Outlook, I'd rather use it than Gmail. I'd rather use basically any chat program as opposed to GChat/Whatever it's called now (it's still really bad whatever the new product is called). Even Google itself I find myself fighting with Google's preferred results instead of what I'm actually wanting (without uBlock the first results are tons of ad spam that have nothing to do with the searches, and the first few results are usually some SEO'd result).
There is an "okay" experience with google if you go all in on it, but they certainly seem to bank on the idea that you'll do that in order to get access to Google results. The rest of the ecosystem is so clunky that it's really undesirable to use for me.