Slow loading pages affects time spend metric?

depends on which part of the "slow" appears.

If it's network level, the coding has not loaded yet, no impact. If it's above network level, the 200 code has responded, then it's up to when the tracking code got fired, as explained by @mattcoen and @frederikwerner 

Others have answered about how time spend on page is calculated.

In terms of page loading you need to figure out what is the best "event" to trigger your page view analytics server call. The later you send it the more likely you will loose visibility on customer reaching your site. 

On traditional pages you would usually use Page Top (when adobe launch is fully loaded), Page Bottom (when the _satellite.pageBottom() is triggered and should be placed just before the closing body tag), DOM loaded and finally Load event. The higher up event you select, the more likely you will see how many visitor reach your page but it will increase time spend on page. However the lower down events will reduce the time spend but might also miss a big portion of visitors coming to the page. It is known that a long loading page will result in customer dropping more frequently, it is therefore a good thing to catch the high time spend on page as you can feedback to your developers the loading issues and make a case to your business to allocate budget to fix loading on your page. 

For SPAs like React apps, you should speak with your developer to trigger the PAGE load event or update the data layer when it is most suited as well. For example some SPA will load shell with spinner and only trigger a page load when the content/components have been fully painted.