Reproduction steps:
1. Download user's attached project and open in Unity
2. Open scene Scene
3. Open BugReport -> Open TestCode Window
4. Press "Capture memory 'Editor'" button several times to simulate a bigger snapshot being captured

Expected:
Receiving a large PackedMemorySnapshot should not cause the editor to freeze for several seconds.
The UnityEditor.MemoryProfiler API should provide a callback, beside "OnSnapshotReceived", to allow reporting the "snapshot receiving progress".
Having functionality to report the progress would smooth out the problem of "OnSnapshotReceived" taking so much time.

Actual: Editor freezes (if the button is pressed more times, the freeze will last longer as the snapshot grows (the later is as expected))

Reproduced on: 2017.3.0a6; 2017.3.0a7; 2017.3.0p1; 2018.1.0b1
Not reproduced on: 2017.2.1p1; 2017.3.0a5
Regression introduced: 2017.3.0a6

Note: Blue rectangle in TestCode window shows the severity of the freeze

Answer:
We are working on improvements on the backend of this so it should get less severe at some point in the near future.

Regarding taking multiple snapshots from the Editor taking longer and longer:
Each snapshot consists of approximately one managed object per recorded managed object. Snapshots taken from the editor will contain the objects in all previous snapshots that have not yet been garbage-collected. Therefore, repeated snapshots of the editor may grow exponentially. If you no longer need an old snapshot it is therefore advisable to null all references to it and call GC.Collect() before taking another one.