Any VPN adds a little overhead, yet a well-tuned one should not slow you down in any way you would notice. When your speeds fall well below normal, work through these steps:
The distance to the VPN server has an impact on speed. In TunnelBear, tap the country selector and pick a country nearer to where you actually are.
Open Settings → Connection and test another protocol. WireGuard is usually the quickest, while OpenVPN UDP makes a dependable backup.
Turn TunnelBear off and run a speed test. A slow underlying connection is something a VPN cannot fix; if it stays slow, get in touch with your ISP.
Quit the app, open it again and reconnect. That clears up most short-lived performance problems.