The crux of the matter is as Maizie said,
The opaqueness of English orthography means that it takes longer to learn to read it
because learning to read with letters that can have different sounds (an, any, apron) is much more difficult than with ones which don't (keep sleep deep). It also means that learning to read English is impossible with just learning to sound out letters and to blend them (i.e. just with phonics).
Teachers who express passionate support for phonics, in practice also have to go over the tricky words (swan swam) over and over again, until children stop stumbling over them - until they can read them by sight.
It simply is not a matter of either / or, but a mixture of phonics and learning to recognise whole words.
This is (as I have said before) because 69 English graphemes have more than one possible pronunciation:
a: and ? apron, any, father
a-e: came ? camel
ai: wait ? said, plait
al: always ? algebra
-all: tall - shall
are: care - are
au: autumn - laugh, mauve
-ate: to deliberate - a deliberate act
ay: stays - says
cc: success - soccer
ce: centre - celtic
ch: chop ?chorus, choir, chute
cqu: acquire - lacquer 19
e: end ? English
-e: he - the
ea: mean - meant, break
ear: ear ? early, heart, bear
-ee: tree - matinee
e-e: even ? seven, fete
ei: veil - ceiling, eider, their, leisure
eigh: weight - height
eo: people - leopard, leotard
ere: here ? there, were
-et: tablet - chalet
eau: beauty ? beau
- ew: few - sew
- ey: they - monkey
ge: gem - get
gi: ginger - girl
gy: gym ? gynaecologist
ho: house - hour
i: wind ? wind down ski hi-fi
- ine: define ?engine, machine
ie: field - friend, sieve
imb: limb ? climb
ign: signature - sign
mn: amnesia - mnemonic
ost: lost - post
-o: go - do
oa: road - broad
o-e: bone ? done, gone
-oes: toes ? does, shoes
-oll: roll - doll
omb: tombola - bomb, comb, tomb
oo: boot - foot, brooch
-ot: despot - depot
ou: sound - soup, couple
ough: bough - rough, through, trough, though
ought: bought - drought
oul: should - shoulder, mould
our: sour - four, journey
ow: how - low
qu: queen ? bouquet
s: sun ? sure
sc: scent - luscious, molusc
-se: rose - dose
ss: possible - possession
th: this - thing
-ture: picture - mature
u: cup ? push
ui: build ? fruit, ruin
wa: was ? wag
wh: what - who
wo: won - woman, women, womb
wor: word ? worn
x: box - xylophone, anxious
- y-: type - typical
- -y: daddy - apply
z: zip ? azure
I am sure that anyone who bothers to take a look at them can easily imagine how much easier learning to read English would be if they had just the one, first shown sound (as they would if English spelling was like other European writing systems).