Time Spent Tier refers to the Time Spent, along with its tiers. Tiers help group users based on the time they spent browsing your website.
Tiers are every 20 seconds:
0-19 seconds
20-39 seconds
40-59 seconds
60-79 seconds
80-99 seconds
100-119 seconds
More than 120 seconds
For example, if a user spent time on your website for 45 seconds, that user would be placed under the 40-59 seconds tier.