Slow transfer speed between 2 Tailscale devices

### What is the issue?

Hello, We are using Tailscale to replicate InfluxDB data from an Edge device to our VM. We now have an issue where we are getting very slow transfer speed. About 24 hours ago, we had 45 mb "Remaining mbytes to by Synced" from our Edge device, now we still have 12 mb to sync.
When connecting to both the Edge device and VM, (both Linux based), from my Windows computer, I'm able to connect directly. However, both the Edge and VM is connected to different DERP relays.

Netcheck from Edge:
"Report:
* UDP: true
* IPv4: yes, xxx.xxx.xxx.xxx:21578
* IPv6: no, but OS has support
* MappingVariesByDestIP: true
* HairPinning: false
* PortMapping:
* CaptivePortal: false
* Nearest DERP: Amsterdam
* DERP latency:
- ams: 66.9ms (Amsterdam)
- par: 67.9ms (Paris)
- fra: 69.1ms (Frankfurt)
- lhr: 72.7ms (London)
- waw: 81.5ms (Warsaw)
- mad: 93.2ms (Madrid)
- nyc: 137.4ms (New York City)
- tor: 145.6ms (Toronto)
- ord: 154.3ms (Chicago)
- dfw: 173.3ms (Dallas)
- mia: 173.4ms (Miami)
- den: 176.8ms (Denver)
- dbi: 179.9ms (Dubai)
- lax: 195.3ms (Los Angeles)
- sea: 198.4ms (Seattle)
- sfo: 203.6ms (San Francisco)
- blr: 211.1ms (Bangalore)
- jnb: 224.5ms (Johannesburg)
- nai: 227.5ms (Nairobi)
- hnl: 245.5ms (Honolulu)
- sao: 253.3ms (São Paulo)
- hkg: 284.2ms (Hong Kong)
- sin: 288.7ms (Singapore)
- tok: 299.5ms (Tokyo)
- syd: 323.4ms (Sydney)"

Netcheck from VM:
"Report:
* UDP: true
* IPv4: yes, xxx.xxx.xxx.xxx:65195
* IPv6: no, but OS has support
* MappingVariesByDestIP: true
* HairPinning: false
* PortMapping:
* Nearest DERP: London
* DERP latency:
- lhr: 31.4ms (London)
- ams: 32.7ms (Amsterdam)
- par: 35.2ms (Paris)
- waw: 44.1ms (Warsaw)
- fra: 48.3ms (Frankfurt)
- mad: 55.2ms (Madrid)
- nyc: 103ms (New York City)
- tor: 113ms (Toronto)
- ord: 119ms (Chicago)
- mia: 133.4ms (Miami)
- dfw: 138.9ms (Dallas)
- den: 139.2ms (Denver)
- dbi: 145.2ms (Dubai)
- blr: 166.7ms (Bangalore)
- sea: 167.2ms (Seattle)
- lax: 170.1ms (Los Angeles)
- sfo: 170.2ms (San Francisco)
- jnb: 180.8ms (Johannesburg)
- nai: 187.3ms (Nairobi)
- hnl: 216.3ms (Honolulu)
- sao: 227.3ms (São Paulo)
- sin: (Singapore)
- syd: (Sydney)
- tok: (Tokyo)
- hkg: (Hong Kong)"

Ping from Edge to VM:
tailscale ping xxx.xxx.xxx.xxx
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 117ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 132ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 109ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(fra) in 112ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(fra) in 107ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 103ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 113ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 102ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(fra) in 104ms
pong from vm (xxx.xxx.xxx.xxx) via DERP(lhr) in 111ms
direct connection not established

Ping from VM to Edge:
tailscale ping xxx.xxx.xxx.xxx
pong from edge (xxx.xxx.xxx.xxx) via DERP(fra) in 108ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(fra) in 111ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(waw) in 106ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(waw) in 139ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(waw) in 111ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(fra) in 111ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(fra) in 141ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(fra) in 101ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(waw) in 119ms
pong from edge (xxx.xxx.xxx.xxx) via DERP(waw) in 101ms
direct connection not established

### Steps to reproduce

_No response_

### Are there any recent changes that introduced the issue?

_No response_

### OS

Linux

### OS version

Debian GNU/Linux 11 (bullseye)

### Tailscale version

1.44.0

### Other software

_No response_

### Bug report

BUG-3913975ab1bf734fefb66e824996eb371fad18cc58ac7fa059568a5a05d2c2d5-20230628090939Z-a87df642043c0792

Start by reviewing the Edge and VM netcheck reports and running the documented tailscale ping checks between them. Investigate why both paths remain on DERP instead of becoming direct, and use the transfer scenario to confirm whether the connection is direct and the sync speed improves.

Written by the indexing model from the issue text.