Byte string catenation inefficient in 3.7?

[RMJFlack]

So Ive had a first step at using numpy. Code is below, Im sure its rather crude and any critique is very welcome.
The time 'signature' in terms of total run time for various n (to give a nxn image) is broadly similar to that for the Python2.7 char string catenation method.

https://gyazo.com/2df2fbef43953fa1c14831abbd63925c

The core code fragments are:

PX = np.zeros((size2, Size), dtype=(int,3))   # size2 is Size rounded up to mult of 4

for Y in range(0, XY[1]): # (BMPs are L to R from the bottom L row)
        for X in range(0, XY[0]):
            # dummy code to make square rainbow, as opposed to real contour data
            x = floor((255 * X)/XY[0])
            y = floor((255 * Y)/XY[1])

            (r,g,b) = [x, y, 128]   #Colour(data[x ,y])
            PX[X,Y] = (b,r,g)

# could probably be a 1 liner
vec = np.ravel(PX, order='C')
vec = [int(v) for v in vec]
vec = bytes(vec)

Here is the new code in full to build the pixel array (a bit messy because f logging & tracking info):

def BuildImage2(name, XY):
    # using numpy array
    global t01

    n0 = 0
    t00 = time.process_time()
    t01 = t00

    def delTime():
        global t01
        t02 = time.process_time()
        t = t02-t01
        t01 = t02
        return t
    
    
    chtName2 = path+'cht_'+name+'2.bmp'
    print("drawing "+chtName2)
    
    hdr = bmpHdr(XY)
    #print(hdr)

    # pretty sure theres a neater way to do this
    row_mod = (hdr['width']*hdr['colordepth']/8) % 4
    if row_mod == 0:
        pad = 0 
    else:
        pad = (4 - row_mod)
    size2 = Size+pad
    
    PX = np.zeros((size2, Size), dtype=(int,3))
    t02 = time.process_time()
    print("empty array tim: {0:.3f} secs".format(delTime()))
    
    for Y in range(0, XY[1]): # (BMPs are L to R from the bottom L row)
        for X in range(0, XY[0]):
            x = floor((255 * X)/XY[0])
            y = floor((255 * Y)/XY[1])
            (r,g,b) = [x, y, 128]   #Colour(data[x ,y])
            PX[X,Y] = (b,r,g)

        #if(0 == Y % 100 or Y == 0):
        #    print("{0:5d} time = {0:6.3f}".format(XY[0]-Y, delTime()))
                
    print("PX populated")
    print( PX.shape)
    t02 = time.process_time()
    print("poulated array tim: {0:.3f} secs".format(delTime()))

    vec = np.ravel(PX, order='C')
    print("PX ravelled, len={0}".format(len(vec)))
    print(type(vec[0]))
    print(vec[0:9])

    vec = [int(v) for v in vec]
    vec = bytes(vec)
    
    print("PX bytes, len={0}".format(len(vec)))
    print(type(vec[0]))
    print(vec[0:9])
    print("bytes array tim: {0:.3f} secs".format(delTime()))

    bmp_write(chtName2, hdr, vec)