Take Galaxy Ace as an example:
It is an Internation/European Android phone with a 3.5-inch screen and 800MHz processor. Samsung describes it as minimal yet sophisticated. Android 2.2 (Froyo) operating system helps this smartphone stand tall next to other mid-range touchscreen devices, such as HTC's Wildfire. Its TFT Color Touch Display with 320 x 480 pixels, the built-in Music Player support MP3/AAC/AAC+/e-AAC+/AMR/MIDI/WAV, Video player back up H.263/H.264/MPEG4 these video formats.

But, it doesn't mean it is perfect and no need to improve. According the market reflection, it not plays all kinds of videos well:
"I have tried many players on this phone but none of them were able to play .mkv files.they lag too badly can anyone suggest something"-from androidforums.com

How to solve it? And make your source files can be playable on the Samsung Galaxy devices without any problem?

Here I would like to recommend a third-party conversion tools-Samsung Galaxy Video Converter which can convert any kinds of video on-line or movie collection in MKV, AVI, MOV, FLV, MPEG, MKV, MOD, TOD, ASF, 3GP, 3G2, M2TS or AVCHD to Samsung Galaxy Ace, Tab 10.1, S2 supported WMV 9, WMV 8, MP4, H.264 playback. You can also convert video to Galaxy Tab 10.1 or Tab 7.7 withSamsung Galaxy Tablet Video Converter.

Firstly, Check out whether your source file can be compatible with Galaxy devices. Learn more about file formats supported by Samsung Galaxy Ace.

Follow the guide on how to convert movies to Samsung Galaxy device format for playing:

Step 1: Free download and install Video Converter for Samsung Galaxy:
Click "Add file" to load videos to the program. For example: you can import YouTube FLV, Facebook MPEG, DVD MPEG-2, QuickTime MOV, Camcorder MOD, AVCHD etc.

Note: If you are running Mac OS X, please refer to Samsung Galaxy Video Converter for Mac.

Step 2: Set Samsung Galaxy output format.
Hit the "Profile" to show the drop-down list, go to "Common Video" then either choose MP4 or WMV((9, 8, 7) as the output format for the tablet.

Tip: Resetting the video resolution.
If your video are SD, you can change it to HD 1080*1028 or 1280*720 resolution here.

Step 4: Start to convert videos to Galaxy MP4 or WMV.
At last, you can either use manual sync way or Samsung software sync way to import the videos to the tablet.